1. Field of the Invention
The present invention relates to a photo mask and a semiconductor device manufacturing method. More particularly, the present invention relates to a phase shifting type of a photo mask, an exposing apparatus, and a semiconductor device manufacturing method, which includes a method of forming a circuit pattern by using the photo mask.
2. Description of the Related Art
In association with a higher integrating degree of a semiconductor integrated circuit, the hyperfine structure of a circuit pattern formed on a semiconductor substrate has been advanced. The hyperfine structure of the circuit pattern causes a line width of a wiring, an electrode or the like to be extremely reduced. Correspondingly to the reduction, a request for a lithography technique has been very strong.
A phase shifting type of a photo mask (a phase shifting mask) is well known as one of the lithography techniques corresponding to the hyperfine structure of the circuit pattern. As the phase shifting mask, there are a Levenson's type in which the phases of lights transmitted through mask openings adjacent to each other on both sides of linear patterns are made opposite to each other, and a half tone type in which a transmission property is given to a light shielding portion, and a phase of a light transmitted through a mask opening is made opposite to a phase of the transmission light through the light shielding portion.
The Levenson's type phase shifting mask will be described below with reference to drawings.
FIGS. 1A to 1D are conceptual views showing the difference between the Levenson's type phase shifting mask and a usual photo mask.
FIG. 1A is a section view of the Levenson's type phase shifting mask (shown on a left side, hereafter similarly in FIGS. 1A to 1D), and the usual photo mask (shown on a right side, hereafter similarly in FIGS. 1A to 1D), and the manner of lights transmitted through each photo mask. FIG. 1B shows an amplitude distribution of the lights immediately after the transmission through each photo mask. FIG. 1C shows an amplitude distribution on a wafer of the lights transmitted through each photo mask. And, FIG. 1D shows an optical magnitude distribution on the wafer of the lights transmitted through each photo mask.
With reference to FIG. 1A, the Levenson's type phase shifting mask 101 has two openings of light shield films 103 formed on a glass substrate 102, and a material referred to as a shifter 104 is placed on one of the openings (besides, it may be shaped so as to be dug into the glass substrate 102 of the opening, instead of the shifter 104). In this case, a light 105 transmitted through the opening without the shifter 104 and a light 106 transmitted through the opening with the shifter 104 are out of phase by 180 degrees from each other (in this specification, referred to as an [Opposite Phase] state).
As shown in FIGS. 1A to 1D, in the Levenson's type phase shifting mask, the lights transmitted through the two openings have the phases ((a), (b)) opposite to each other. For this reason, when they arrive at the wafer and the lights are spread out, even if their feet overlap with each other (c), there is always a region of a magnitude of zero between them. Thus, there is no case that they strengthen each other (d). Hence, it is possible to resolve a linear pattern sandwiched between the two openings precisely and accurately. In addition, a focal depth can be improved.
On the other hand, in a usual photo mask 111, nothing is placed on two openings of light shield films 113 formed on a glass substrate 112 (and nothing is dug). In this case, the lights transmitted through the respective openings are at a state at which their phases are equal to each other (an in-phase state).
As shown in FIGS. 1A to 1D, in the usual photo mask, the lights transmitted-through the two openings have the phases ((a), (b)) equal to each other. Then, when they arrive at the wafer, the lights are spread out. Their feet overlap with each other (c), and they strengthen each other (d). Thus, it is difficult to resolve a linear pattern (a wiring pattern and the like) sandwiched between the two openings precisely and accurately.
On the other hand, in a photo lithography, it is known that the roughness and the fineness of a pattern cause a dimension of a latent image (hereafter, referred to as [Transfer Pattern]) formed on a semiconductor substrate to be different even under the same mask pattern dimension (a proximity effect). In particular, the optical magnitude distributions between lines and spaces pattern and an isolated pattern are different in a case of the same exposure amount. Here, the lines and spaces pattern is a pattern in which linear patterns are arranged cyclically and densely (hereafter, referred to as [Dense Pattern]). Also, the isolated pattern is a pattern in which a distance from a different pattern adjacent thereto is separated to a degree that a mutual influence can be ignored from the viewpoint of a lithography, and it is separated from the adjacent pattern by at least two times or more of a usual line width, and it is desired to be separated by three times or more.
For this reason, for example, when the exposure amount is adjusted such that the dense pattern is resolved in accordance with a design, this results in a problem that the isolated pattern is deviated from a designed dimension.
For this reason, when a photo mask to transfer the circuit pattern in which the isolated pattern and the dense pattern are mixed is used, the following method is employed. That is, this is the method of preliminarily providing a compensation (hereafter, referred to as [Mask Bias]) for the designed dimension of the isolated pattern of the photo mask and thereby avoiding the deviation in the designed dimension of the isolated pattern induced in the case when the dense pattern is exposed under the exposure condition in which it is resolved in accordance with the design.
That method will be described below in detail.
FIGS. 2A to 2D show a photo mask 210 (a plan view of FIG. 2A and an A–A′ section view of FIG. 2B) used at a first step in an exposure to transfer a circuit pattern in which an isolated pattern and a dense pattern are mixed, and a photo mask 211 (a plan view of FIG. 2C and an A–A′ section view of FIG. 2D) used at a second step.
Also, FIGS. 3A and 3B show a pattern (FIG. 3A) transferred by using the photo mask 210 and a pattern (FIG. 3B) transferred by using the photo mask 211 after that.
In FIG. 2A, the photo mask 210 is the Levenson's type phase shifting mask. It is formed by forming a light shield film 202 exemplified as Cr film on a glass substrate 201 exemplified as quartz glass and then patterning the light shield film 202 and the glass substrate 201. It has an isolated pattern region 205 to transfer an isolated pattern and a dense pattern region 206 to transfer a dense pattern.
The isolated pattern region 205 has an opening 212 and an opening 213. The opening 212 and the opening 213 are defined such that the phases of their transmission lights are opposite to each other (an opening 213 is dug, as shown in FIG. 2B). Then, an isolated pattern 214 between the opening 212 and the opening 213 is transferred.
On the other hand, the dense pattern region 206 has openings 222 (a, b) and openings 223 (a, b). The openings 222 (a, b) and the openings 223 (a, b) are defined such that the phases of their transmission lights are opposite to each other (the openings 223 (a, b) are dug, as shown in FIG. 2B). Then, dense patterns 224 (a, b, c) sandwiched between the openings 222 (a, b) and the openings 223 (a, b) are transferred.
Consequently, a transfer pattern A 208 as shown in FIG. 3A which has the size contracted from the photo mask 210 by a mask magnification is formed on a photo resist layer on a wafer (a semiconductor substrate). In FIG. 3A, the patterns corresponding to the opening 212 and the opening 213 are an open pattern 232 and an open pattern 233. Then, an isolated pattern 214 is transferred to a transfer isolated pattern 234. Also, the patterns corresponding to the openings 222 (a, b) and the openings 223 (a, b) are open patterns 242 (a, b) and open patterns 243 (a, b). Then, the dense patterns 224 (a, b, c) are transferred to transfer dense patterns 244 (a, b, c).
Next, in FIG. 2C, the photo mask 211 is the usual photo mask. It is formed by forming the light shield film exemplified as the Cr film on the glass substrate 203 exemplified as the quartz glass, and then patterning the light shield film. A light shield isolated pattern 216 optically shields a region generated by the open pattern 232, the transfer isolated pattern 234 and the open pattern 233. Light shield end patterns 217 (−1, 2) optically shield a region constituting ends (219 (−1, 2) of FIG. 3B) of a final circuit pattern (a transfer isolated pattern 220). Light shield dense patterns 226a, 226b and 226c optically shields a region generated by the open pattern 242a, the transfer dense pattern 244a and the open pattern 243a, a region generated by the open pattern 243a, the transfer dense pattern 244b and the open pattern 242b, and a region generated by the open pattern 242b, the transfer dense pattern 244c and the open pattern 243b, respectively. Light shield end patterns 227 (−1, 2) (a, b, c) optically shield a region constituting ends (229 (−1, 2) (a, b, c) of FIG. 3B) of a final circuit pattern.
Consequently, a transfer pattern B 209 as shown in FIG. 3B which has the size contracted from the photo mask 211 by a mask magnification is formed on a photo resist layer. In FIG. 3B, the pattern corresponding to the isolated pattern 214 of FIG. 2A is the transfer isolated pattern 220. The pattern corresponding to the light shield end pattern 217 of FIG. 2B is the transfer end pattern 219. Also, the patterns corresponding to the dense patterns 224 (a, b, c) of FIG. 2A are transfer dense patterns 230 (a, b, c). The patterns corresponding to the light shield end patterns 227 (a, b, c) of FIG. 2B are the transfer end patterns 229 (a, b, c).
In the above-mentioned processes, in the photo mask 210 to transfer the circuit pattern in which the isolated pattern and the dense pattern are mixed, the mask bias is preliminarily performed on the isolated pattern 214. That is, the dimension of the isolated pattern 214 on the photo mask is compensated in order to attain the dimension of the transfer isolated pattern 220 in accordance with the design.
This compensation will be described below with reference to FIG. 4.
FIG. 4 is an example of a graph showing a relation between a line width of a linear pattern and a distance between lines adjacent to each other (an inter-line distance), when the Levenson's type phase shifting mask is used. Its horizontal axis indicates a distance (hereafter, referred to as [Inter-Line Distance], nm) between linear patterns (hereafter, referred to as [Linear Transfer Pattern]) transferred to a photo resist layer, and its vertical axis indicates a line width (nm) of the linear transfer pattern. Here, a1, a2, a3, b1, A and B will be described with reference to FIG. 5.
Here, the line width of a targeted linear transfer pattern is designed as 100 nm, and the inter-line distance is designed as 250 nm. Then, let us consider the case when a light of an exposure amount and a photo mask from which the transfer pattern in accordance with the design can be obtained are used (indicated by 250 nm EOP in FIG. 4). On the photo mask, the pattern is usually formed at the size equal to several times the transfer pattern (hereafter, referred to as [Mask Magnification]). Then, if the distance between the patterns on the photo mask is changed in the above-mentioned photo mask, the inter-line distance of the transfer pattern is changed (the horizontal axis) In association with the change, the line width of the transferred pattern is changed (the vertical axis).
From this graph, in the above-mentioned condition, when the inter-line distance of the pattern on the photo mask is changed (however, the line width of the pattern on the photo mask is not changed) such that the interline distance on the transfer pattern becomes 150 nm, the line width on the transfer pattern becomes 75 nm. Thus, if a compensation of (+25(=100−75) nm×a mask magnification) is provided for the line width of the pattern on the photo mask (the increase in the line width of 25%), the line width after the transfer is expected to be 100 nm. Also, in a case when the inter-line distance is 300 nm, the line width on the transfer pattern is 115 nm. Hence, if a compensation of (−15(=100−115) nm×a mask magnification) is provided for the line width of the pattern on the photo mask (the decrease in the line width of 15%), the line width after the transfer is expected to be 100 nm.
The above-mentioned compensation is referred to as a mask bias. In the conventional process, the compensation for the isolated pattern is carried out in accordance with the above-mentioned theory.
However, in the actual compensation, the relation between the change amount of the line width on the photo mask and the change amount of the line width of the transfer pattern largely depends on the inter-line distance. It will be described below with reference to FIG. 5.
FIG. 5 is a graph showing the relation between a compensation amount of a line width of a photo mask and a compensation amount (a change amount) of a line width of a transfer pattern, in the condition of FIG. 4. Its horizontal axis is a photo mask compensation amount (nm)=a compensation amount of a line width of a pattern in a photo mask/a mask magnification. Its vertical axis is a line width compensation amount (nm)=a change amount (a compensation amount) of a line width changed (compensated) in a transfer pattern, when a pattern on a photo mask is compensated. In the graph, the curved lines a1, a2, a3 and b1 correspond to the points a1, a2, a3 and b1 in FIG. 4. The dashed line shows a case of a photo mask compensation amount=a line width compensation amount. This graph can be determined by an experiment or a simulation.
From FIG. 5, it is known that the slopes of the curved lines are gentle in the curved lines a1, a2 and a3. For example, in the curved line al (the inter-line distance is 400 nm in FIG. 4), even if the compensation corresponding to 200 nm with respect to the photo mask compensation amount is done, the line width compensation amount is 5 nm. With regard to this slope of the curved line, let us consider the following index MEF (Mask Error Factor).MEF=Line Width Compensation Amount/Photo Mask Compensation Amount  (1)This corresponds to the slope of each curved line of FIG. 5. That is, in the curved lines a1, a2 and a3, MEF is low. In the case of the curved line a1, MEF (a1)=0.25. Thus, if a compensation of 20 nm (=a line width compensation amount of 20 nm) is desired to be done in the transfer pattern, the photo mask compensation amount becomes 80 nm. This implies that the photo mask requires the compensation of 320 nm multiplied by the mask magnification (assumed to be 4).
On the other hand, in the case of the curved line b1 (the inter-line distance of 200 nm in FIG. 4), MEF (b1)=1.25. Thus, if the compensation of 20 nm (=the line width compensation amount of 20 nm) is desired to be done in the transfer pattern, it is enough to carry out the compensation corresponding to 16 nm, on the photo mask. Then, in the photo mask, it is enough to carry out the compensation of 64 nm multiplied by the mask magnification (assumed to be 4).
As mentioned above, in the Levenson's type phase mask, the compensation amount is largely different depending on the inter-line distance in the transfer pattern. Because of the restriction on the design of the photo mask, the restriction on the exposing apparatus and the like, there is the compensable range (the upper limit on the photo mask compensation amount). The illustrated range is the regions represented by A, B of FIG. 4. In the region represented by B, the inter-line distance is short, and it provides the effect of the phase mask. Thus, since the compensation amount is also low, the compensation is possible. However, in the region represented by A, the inter-line distance is long. Thus, the effect of the phase mask is poor, and the compensation amount becomes high. Hence, because of the above-mentioned restrictions, the compensation is impossible.
This implies that since the transfer pattern having the long inter-line distance (in this example, the inter-line distance of 300 nm or more) belongs to the A region, the hyperfine pattern of 100 nm or less can not be formed even if the effect of the Levenson's type phase mask is used. That is, the hyperfine pattern can not be formed in the isolated pattern (=the transfer pattern having the long inter-line distance).
As the related technique, Japanese Laid Open Patent Application (JP-A-Heisei 11-283904) discloses a technique of an exposing method. This technique is the exposing method of forming a latent image of a photo resist by carrying out a plurality of exposures including a high resolution exposure and a usual exposure. In the high resolution exposure, a pattern of a portion having a severe line width control property is transferred to a photo resist layer by using a phase shifting pattern. In the usual exposure, while the portion of the photo resist layer to which a pattern was already transferred through the high resolution exposure is protected by using a light shield portion of a mask pattern, a pattern of a portion having a relatively loose line width control property is transferred to the photo resist layer without any usage of the phase shifting pattern. This exposing method is used to form the latent pattern of the photo resist. However, the high resolution exposure employs the exposure condition that the transfer pattern line width after the high resolution exposure is performed on the portion having the severe line width control property is thicker than a desired line width. Then, the desired line width is obtained in the portion having the severe line width control property after the usual exposure.
That is, this is a two-stage (multiple) exposing method, as described below. For the pattern in which the line width control is difficult in the Levenson's type phase shifting mask such as the isolated pattern, a slightly thick transfer pattern is formed in the high resolution exposure using a first Levenson's type phase shifting mask. Then, the desirably slightly slender transfer pattern is formed in the usual exposure using a second usual photo mask.
In this case, since the final exposure is the usual exposure, it is influenced by the focal depth and the resolution of the usual exposure. Thus, in the exposure for transferring the circuit pattern in which the isolated pattern and the dense pattern are mixed, the formation of the hyperfine isolated pattern is considered to be difficult.
Also, Suzuki et al. announces the following technique for the pattern in which the line width control is difficult in the Levenson's type phase shifting mask such as the isolated pattern. At first, the Levenson's type phase shifting mask in which an isolated pattern and a cyclic pattern around it are formed is used to carry out a first exposure in a weak light. After that, the usual photo mask in which only the isolated pattern is formed is used to carry out a second exposure in a weak light. At this time, only the photo resist layer of the portion of the isolated pattern receives the lights corresponding to the two exposures. The magnitude of the weak light does not have the magnitude required to resolve under the light corresponding to only one exposure. However, it is set so as to have the magnitude required to resolve under the lights corresponding to the two exposures. Thus, only the photo resist layer of the portion of the isolated pattern receiving the lights corresponding to the two exposures is resolved. At this time, as for the material for the photo resist layer, the material suitable for it is also selected. (A. Suzuki et al., “Multilevel imaging system realizing k1=0.3 lithography”, Proceedings of SPIE Optical Microlithography SPIE, 3679, (1999) pp. 396–407).
That is, the magnitudes of the lights for the exposures to be done two times and the sensibility of the photo resist layer are set so as to comply with the above-mentioned conditions. Thus, the exposures of the two times are done to thereby resolve the photo resist layer into the desirable pattern.
In this case, the stability of the magnitude of the exposing light, the characteristic regularity of the photo resist layer, the uniformity of a film thickness, the property of the photo resist material and the like influence each other. Thus, the technical difficulty is expected in view of the optimality of the condition and the reservation of the reliability.
Japanese Laid Open Patent Application (JP-A-Heisei 10-10700) discloses a technique of a photo mask and its manufacturing method.
The photo mask of this technique is a half tone phase shift type photo mask for transferring fine patterns which comprises a isolated pattern portion and a periodical pattern portion, and transfers fine patterns by using an interference of refracting lights at the periodical pattern portion. The isolated pattern portion includes a first opening fabricated in a semitransparent film covering with a transparent substrate. The periodical pattern portion includes a plurality of second openings periodically fabricated in the area separated from the area having the isolated portion by a certain distance in the semitransparent film. The thickness of the semitransparent film is λ(n−1)/2. Here, λ is a wave length of a exposing light for transferring patterns, and n is refraction rate. The photo mask included a assistant pattern having a light shielding film with an width of W and separated from the first opening by the distance of L, and 0.2λ/NA<L, W<1,3λ/NA. Here, NA is a number of openings of an exposing apparatus
Japanese Laid Open Patent Application (JP-A-Heisei 9-73166) discloses a technique of a photo mask for an exposure and its manufacturing method.
The photo mask of this technique is a photo mask for an exposure that includes a pattern made by a light shielded film on the transparent substrate, and has a main pattern and an assistant pattern. The main pattern, which is transferred by using a projection exposure method on a surface of a semiconductor substrate, is made by the light shielded film. The assistant pattern, which is made by a film having a low refraction rate against a projection light for an exposure, is arranged around the main pattern.